The present invention relates to wireless signal transmission systems, methods and apparatus, for example, radio transmission apparatus for transmitting audio signals within a local transmission area to a portable radio receiver means carried on the person of the user.
Personal wireless audio signal transmission apparatus include systems which transmit audio signals, such as television audio signals, by means of infrared light received by a personal infrared light receiving device worn by a listener. It will be appreciated that such transmission systems require a line-of-sight transmission path, so that the system is not workable if walls, furniture or other objects intervene between the transmitter and receiver. Accordingly, while infrared transmission systems may be useful where, for example, a person is seated several feet from a television receiver to which the infrared transmitter is connected for transmitting television sound, the transmission path may be interrupted if, for example, the listener turns his or her head away from the transmitter or a person walks between the transmitter and the listener. Moreover, it is not practical to utilize an infrared transmission system where, for example, the listener is positioned in another room or outside a building in which the transmitter is located.
Local wireless television transmission systems are available which transmit television signals from a local source, such as a television or VCR, within the 900 MHz local television transmission band to a receiver which downconverts the television signals to a frequency band which may be tuned by a conventional television receiver. Such systems, therefore, employ receivers which are designed for use with a stationery television set and which optionally utilize a directional antenna carefully positioned for best reception of the 900 Mhz signals radiated by the local transmitter. It is desirable, therefore, that the receiver act as a stable base for supporting the receiving antenna in the best disposition to receive the locally transmitted signal, and therefore, the receiver is typically of a size and weight not practical for carrying on the person of a listener.
Many television stations now include stereo audio signals in their transmissions. It is, therefore, desirable that a personal wireless audio signal transmission apparatus provide the capability of transmitting stereo audio signals reproduced by a television receiver. Conventionally, stereo audio signals are formed by adding the right and left audio channels to form a first signal and subtracting the right and left channels to form a second signal which is modulated on a subcarrier of 38 kilohertz. The subcarrier is suppressed and the combination of the first signal, the subcarrier suppressed modulated second signal and a pilot signal having a frequency of 19 kilohertz (one-half that of the subcarrier), constituting a multiplexed stereo signal, modulates a carrier for transmission. Conventional integrated circuits for producing such multiplexed stereo signals are available commercially.
However, audio signals provided by a television receiver typically contain unwanted components at the horizontal frequency of the video signal (approximately 15,734 kilohertz in an NTSC signal) and harmonics thereof. Applicants have found that the use of a 38 kilohertz subcarrier to form the multiplexed stereo signal causes mixing with the second harmonic of the NTSC signal, resulting in audible beat interference. In an attempt to overcome this problem, applicants have instead employed a subcarrier having a frequency equal to two times the horizonal frequency of the video signal, approximately 31.5 kilohertz. However, similar beat interference problems resulted. Applicants further attempted to overcome this problem with the use of a subcarrier equal to four times the horizontal video frequency, but were unsuccessful due to a loss of stereo separation resulting from the use of an excessively high subcarrier frequency.
With the introduction of digital audio recording media, such as compact discs, and digital reproduction techniques, the ability to reproduce high quality audio signals having superior frequency response and wide dynamic range requires the provision of a similarly capable personal wireless transmission system. The transmitter of such a system must be capable of modulating a carrier without introducing audible distortion at the receiver, for example, due to overmodulation. Transmitters typically employ an overmodulation detector which provides a visual indication when the level of the modulating signal is excessive, thus to enable a user to avoid overmodulation distortion while maintaining a desirably high signal-to-noise ratio.
Conventional overmodulation detectors utilize a threshold detector whose output changes state when a level of a modulation signal exceeds a predetermined threshold level, and resumes a prior state once the level of the modulation signal falls below the predetermined threshold level. The output of the threshold detector is used to drive a visual indicator, such as an LED. However, modulation signals which exceed the threshold level for only brief intervals might not produce a visible indication by the conventional apparatus. This becomes especially troublesome where the modulation signal is supplied by a source such as a compact disc player which can produce an output signal having much sharper peaks than typical analog reproduction devices such as a phonograph or magnetic tape recorder. Accordingly, it is possible that a conventional overmodulation detector will be unable to provide a visible indication of sharp peaks in the modulation signal, such as those provided by a compact disk player, with the result that objectionable overmodulation distortion is audible at the receiver, but not detectable by the overmodulation detection circuit.
It will be readily appreciated that a personal wireless audio receiver must be battery operated in order to permit mobility of the person while the receiver is in use. However, the need to replace worn out batteries from time to time is a nuisance, so that it is desirable to employ rechargeable batteries to power a personal wireless audio receiver. It is also inconvenient, however, to remove rechargeable batteries for recharging and subsequently reinstall the same. In addition, the user may find that the batteries need recharging when it is desired to resume use of the receiver, which is also inconvenient.